The invention relates to a conductive paste for the electrical industry, and also to its use.
Materials of this type are widely used and are required by a wide variety of applications. They have gained particular importance, for example, in connection with the sealing of electromagnetically shielded housings in electronic devices which emit electromagnetic radiation or can be disturbed by electromagnetic radiation penetrating from outside. To give EMI (electromagnetic interference) shielding or, respectively, RFI (radio frequency interference) shielding, and to improve electromagnetic compatibility (EMC), the housings are produced from a material which is electrically conducting or has been coated with an electrical conductor. It is known that gaskets made from an electrically conductive flexible material can be used so that the region of the joints at which the parts of the housing are joined together is also given shielding.
An example of a material of this type is known from U.S. Pat. No. 4,011,360. This known material is based on an elastomer, specifically a silicone rubber material which has an admixture of electrically conducting particles. This material polymerizes when exposed to atmospheric moisture at room temperature.
DE 43 19 965 C2 discloses the use of a material of this type for producing the housings described at the outset. The starting material is extruded as a strand of paste directly in the region of the joint onto one of the parts of the housing, and is polymerized there to form the gasket. This process is also known by the skilled worker as the formed-in-place-gasket process.
Materials of this type with electrically conducting particles are also used to form contact points or contact areas, known as contact pads, thus taking on the function of contact elements.
The electrically conductive particles may have various shapes. Particles frequently encountered have the shape of flakes, spheres, irregularly shaped bodies or fibers. In the simplest case, the particles are manufactured directly from conducting material. It is moreover possible to prepare particles from nonconducting materials and then to cover or coat these with conductive material. A factor common to these particles is that they have a given shape and therefore reduce the elasticity of the cured final product. The elasticity here is given solely by the elastomer into which the particles have been embedded.
To achieve good conductivity it is generally necessary to provide more than 50% by volume of conductive particles to ensure the required contact of the particles with one another. The resultant proportion of elastomer is comparatively small and leads to a rigid final product with disadvantageous mechanical properties. These materials are moreover very expensive, since the price of the particles is high.
The object of the present invention is to provide a material of the type mentioned at the outset in which the problems described have been overcome. The material provided should have improved mechanical properties and moreover be inexpensive, in order for example to open up even those application sectors in which economic reasons have hitherto prevented the use of large elastomer gaskets.
The invention is based on the concept of using conductive particles in the form of fibers which are flexible. These are admixed in random orientation with the elastomer in a way which produces a large number of contact points. The embedding of the fibers in the elastomer therefore takes the form of an irregular three-dimensional matrix. The contact points ensure ideal and statistically random distribution conductivity within the material.
Depending on the nature of the fibers used, one preferred embodiment allows the selected proportion of fibers to be less than 50%, based on the total volume of the material. It is preferably possible to reduce the proportion of fibers to less than 25% by volume. Compared with materials previously available, this gives a significant improvement in mechanical properties, in particular in the flexibility of the polymerized final product, which is moreover very inexpensive.
It is preferable for the fibers used to have a length/diameter ratio of more than 2. Ideal results can be achieved if the length/diameter ratio is more than 10. The shape of fibers of this type makes them highly flexible, and they therefore give an ideal result with regard to the mechanical properties of the final product.
An embodiment further optimized in this respect provides for the use of fibers with a diameter of less than 0.1 mm. These extremely thin fibers can be embedded into the elastomer in an ideal manner, such that the embedded fibers have virtually no adverse effect on the flexibility of the elastomer.
It is also possible to design the material in a manner known per se as a two- or multicomponent material. The two components are not mixed until immediately prior to processing. This method can give a very inexpensive material which is easy to process.
Depending on the application, the material selected here may be one which polymerizes at room temperature. On the other hand it is also possible to provide a material which polymerizes on exposure to heat, so that the polymerization procedure can be controlled. This is of particular importance with a view to automated mass production.
The formulation of the material is preferably such that it has low viscosity. It is therefore particularly suitable for forming an electrically shielding gasket for a housing, the material for the gasket being applied in the form of a strand directly in the region of a joint of a housing. Typical applications for a material of this type are the formation of a gasket for a mobile telephone housing or the like.
It is also possible to design a material with high viscosity, for example in order to produce sealing elements in the form of sealing strips, sealing pads, sealing tubes or O-rings, by injection molding or extrusion.
One specific example of an application provides for the use of the conductive paste for producing a flexible gasket for an electromagnetically shielded housing. A paste of the material is applied by means of a path-controlled nozzle directly to a part of the housing in the region where the housing has a joint to be sealed. The nozzle is moved over the part of the housing by means of computer-controlled metering equipment while the plastics material is being discharged. The velocity of relative movement of the nozzle and the part of the housing is determined by the viscosity of the paste, the amount and velocity of the paste emerging from the nozzle, the cross-sectional area of the nozzle passage, the desired cross-sectional profile of the gasket to be produced and the makeup of the material.
The strand made from the paste and dispensed in this way polymerizes under ambient conditions at room temperature. This procedure takes a relatively long time, but can be accelerated by controlled exposure to heat.